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1 /* LibTomMath, multiple-precision integer library -- Tom St Denis |
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2 * |
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3 * LibTomMath is a library that provides multiple-precision |
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4 * integer arithmetic as well as number theoretic functionality. |
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5 * |
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6 * The library was designed directly after the MPI library by |
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7 * Michael Fromberger but has been written from scratch with |
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8 * additional optimizations in place. |
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9 * |
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10 * The library is free for all purposes without any express |
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11 * guarantee it works. |
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12 * |
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13 * Tom St Denis, [email protected], http://math.libtomcrypt.org |
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14 */ |
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15 #include <tommath.h> |
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16 |
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17 /* single digit subtraction */ |
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18 int |
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19 mp_sub_d (mp_int * a, mp_digit b, mp_int * c) |
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20 { |
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21 mp_digit *tmpa, *tmpc, mu; |
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22 int res, ix, oldused; |
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23 |
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24 /* grow c as required */ |
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25 if (c->alloc < a->used + 1) { |
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26 if ((res = mp_grow(c, a->used + 1)) != MP_OKAY) { |
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27 return res; |
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28 } |
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29 } |
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30 |
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31 /* if a is negative just do an unsigned |
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32 * addition [with fudged signs] |
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33 */ |
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34 if (a->sign == MP_NEG) { |
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35 a->sign = MP_ZPOS; |
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36 res = mp_add_d(a, b, c); |
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37 a->sign = c->sign = MP_NEG; |
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38 return res; |
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39 } |
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40 |
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41 /* setup regs */ |
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42 oldused = c->used; |
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43 tmpa = a->dp; |
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44 tmpc = c->dp; |
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45 |
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46 /* if a <= b simply fix the single digit */ |
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47 if ((a->used == 1 && a->dp[0] <= b) || a->used == 0) { |
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48 if (a->used == 1) { |
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49 *tmpc++ = b - *tmpa; |
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50 } else { |
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51 *tmpc++ = b; |
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52 } |
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53 ix = 1; |
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54 |
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55 /* negative/1digit */ |
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56 c->sign = MP_NEG; |
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57 c->used = 1; |
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58 } else { |
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59 /* positive/size */ |
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60 c->sign = MP_ZPOS; |
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61 c->used = a->used; |
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62 |
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63 /* subtract first digit */ |
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64 *tmpc = *tmpa++ - b; |
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65 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1); |
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66 *tmpc++ &= MP_MASK; |
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67 |
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68 /* handle rest of the digits */ |
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69 for (ix = 1; ix < a->used; ix++) { |
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70 *tmpc = *tmpa++ - mu; |
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71 mu = *tmpc >> (sizeof(mp_digit) * CHAR_BIT - 1); |
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72 *tmpc++ &= MP_MASK; |
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73 } |
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74 } |
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75 |
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76 /* zero excess digits */ |
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77 while (ix++ < oldused) { |
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78 *tmpc++ = 0; |
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79 } |
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80 mp_clamp(c); |
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81 return MP_OKAY; |
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82 } |
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83 |
